The present invention relates to a Cu—In—Ga—Se (hereinafter referred to as “CIGS” where appropriate) quaternary alloy sputtering target that is used upon forming a CIGS quaternary alloy thin film to become a light-absorbing layer of thin-film solar cells.
In recent years, the technical development of CIGS-based solar cells with high conversion efficiency for use as thin-film solar cells has advanced. As a method of producing the light-absorbing layer of such thin-film solar cells, the vapor-deposition technique and the selenization method are known. Nevertheless, the solar cells produced via the vapor-deposition technique are advantageous of having high conversion efficiency, but have drawbacks; namely, low deposition rate, high cost, and low productivity.
Meanwhile, the selenization method is suitable for industrial mass production, but a complex and dangerous process of performing heat treatment and selenization in a selenium hydride atmosphere gas is required after preparing a laminated film of Cu—Ga and In, and there are drawbacks in terms of cost and time.
Thus, attempts are being made for preparing a CIGS quaternary alloy light-absorbing layer in a single sputtering process by using a CIGS quaternary alloy sputtering target. At present, a CIGS quaternary alloy sputtering target suitable for forming a light-absorbing layer has not yet been achieved.
Patent Document 1 discloses a method of producing a CIGS quaternary alloy sputtering target by: using a Cu molten metal as the starting material; adding Se thereto to prepare a Cu—Se-based binary alloy molten metal; subsequently placing In therein to prepare a Cu—Se—In alloy molten metal; thereafter placing Ga therein to prepare a Cu—Se—In—Ga alloy molten metal; solidifying this to form a CIGS quaternary alloy ingot; thereafter subjecting the ingot to dry grounding; and then hot-pressing the pulverized powder.
Nevertheless, with the CIGS quaternary alloy sputtering target obtained based on the foregoing production method, Patent Document 1 fails to clarify the target structure, film characteristics after being deposited, density, oxygen concentration, and other matters which are issues during sputtering.
Patent Document 2 describes that a Cu—Se powder, a Cu—In powder, a Cu—Ga powder and a Cu—In—Ga powder are mixed and subject to hot pressing, and lays emphasis on safely producing a target for deposition of a chalcopyrite-type semiconductor, but does not in any way clarify the target structure, film characteristics after being deposited, density, oxygen concentration and other matters.
Moreover, Non-Patent Document 1 discloses a method of producing a CIGS quaternary alloy sputtering target by preparing mechanical alloy powder to become the nanopowder raw material, and thereafter performing HIP (Hot Isostatic Pressing) treatment thereto, and further discloses the characteristics of the obtained target.
Nevertheless, with respect to the characteristics of the CIGS quaternary alloy sputtering target obtained based on the foregoing production method, Non-Patent Document 1 qualitatively describes that the density was high, but fails to clarify any specific numerical values of the density.
Moreover, while it is anticipated that the oxygen concentration is high since nanopowder is being used, Non-Patent Document 1 also fails to clarify the oxygen concentration of the sintered compact. In addition, since expensive nanopowder is being used as the raw material, it is inappropriate as a solar cell material which is demanded of low cost.
Moreover, Non-Patent Document 2 discloses a sintered compact having a composition of Cu(In0.8Ga0.2)Se2, density of 5.5 g/cm3, and relative density of 97%. Nevertheless, there is the only description about its production method that the uniquely synthesized raw material powder is sintered via hot press, and the specific production method is not indicated. Moreover, Non-Patent Document 2 also fails to provide any description regarding the oxygen concentration of the obtained sintered compact, target structure, and film characteristics after being deposited.
Non-Patent Document 3 describes technology for preparing a CuIn0.7Ga0.3Se2 alloy sputtering target by subjecting the respective powders of Cu, In, Ga, and Se to mechanical alloying, and additionally performing hot isostatic pressing thereto at 750° C. and 100 MPa. Non-Patent Document 3 describes that the crystal grain size is approximately 50 nm. Nevertheless, Non-Patent Document 3 does not in any way describe the target structure and film characteristics after being sputter-deposited.
Non-Patent Document 4 describes technology for producing a CIGS target by using a powder raw material of CuIn0.72Ga0.28Se2 and subjecting this powder to hot press. Non-Patent Document 4 describes that it is thereby possible to produce a high-density target, and that the structure of this CIGS target contains Cu2Se, In2Se3, and Ga2Se3.
Nevertheless, what becomes a problem in the target structure is the existence of numerous heterogenous phases other than the Cu(In, Ga)Se2 phase. The existence of this kind of heterogenous phase structure induces the deterioration of the conversion efficiency. Non-Patent Document 4 has not recognition of this fact.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2008-163367    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2009-287092    [Non-Patent Document 1] Thin Solid Films 332 (1998) 340-344    [Non-Patent Document 2] November 2009 issue of Electronic Materials, pages 42 to 45    [Non-Patent Document 3] Written by C. Suryanarayana and 4 others, “Synthesis and processing of a Cu—In—Ga—Se sputtering target” Thin Solid Films 332 (1998) 340-344    [Non-Patent Document 4] Written by Zhang Ning and 2 others, “An investigation on preparation of CIGS targets by sintering process”, Materials Science and Engineering B 166 (2010) 34-40